1. Industrial Field of the Invention
The present invention relates to a booster of the gaseous pressure type for outputting an assisted brake operating force in a vehicle or the like.
2. Related Art
Conventionally, a booster of the gaseous pressure type used in assisting a brake force by employing a large differential pressure between a negative pressure and a compressed gaseous pressure is known for use in a vehicle in which a motor can be employed to provide a negative pressure source and in which a powerful brake force is required. A booster of the gaseous pressure type for assisting a brake force by employing a differential pressure between the air pressure and a compressed gaseous pressure is also known for use as a booster of the gaseous pressure type used in a vehicle in which a motor cannot be employed as a negative pressure source, e.g., for a vehicle driven by a diesel engine, an electric motor or the like, and in which a powerful brake force is required. An example of such a booster of the gaseous pressure type is disclosed in Japanese Patent Application, First Publication, Laid-Open No. Hei 2-6260.
The booster of the gaseous pressure type comprises: a movable partition which divides an inside of a shell into a constant pressure chamber communicating with a negative pressure source, and a variable pressure chamber; a valve body which is provided on the movable partition and into which compressed air is introduced from the side opposite to the constant pressure chamber thereof; an output shaft and a return spring which are provided in the side of the constant pressure chamber of the valve body; an input shaft provided in the side opposite to the constant pressure chamber of the valve body; and a valve plunger having a side of the constant pressure chamber for receiving a reactive force from the output shaft and a side of the input shaft which has a sealed slidable portion on the valve body, wherein the valve plunger is slidably provided in the valve body and engaging the input shaft, and the side opposite to the constant pressure chamber of the plunger faces the compressed air. A first valve seat is provided on the valve plunger and a second valve seat is provided on the valve body. A valve member is provided on the valve plunger. A force to bring the valve member into contact with the first valve seat is applied to the valve member by a force-imparting member. The valve member cuts off communication between the variable pressure chamber and the compressed air source by seating in the first valve seat, and cuts off communication between the constant pressure chamber and the variable pressure chamber by seating in the second valve seat. A valve plunger return spring which applies a force to separate the valve member from the second valve seat, via the input shaft and the valve plunger engaging the input shaft, is provided between the input shaft and the valve body.
When no input force is applied from the brake pedal, the valve plunger is brought into contact with the valve member by the force applied by the force applying member to cut off the communication between the variable pressure chamber and the compressed air source, and the valve member is separated from the valve body by a force from the valve plunger return spring in order for the variable pressure chamber to communicate with the constant pressure chamber.
In the above-described state, a large compressed gas pressure from the compressed air source is exerted on a side of the valve plunger opposite to the constant pressure chamber. Therefore, the force imparted by the valve plunger return spring must be large in order to separate the valve member from the valve body in opposition to the compressed gaseous pressure. Consequently, there are problems in that the space required for providing the valve plunger return spring is large and the entirety of the booster is large.